JP7693583B2 - Rare earth magnet and its manufacturing method - Google Patents

Description

The present disclosure relates to a method for producing a rare earth magnet. In particular, the present disclosure relates to a method for producing a rare earth magnet that contains Sm, Fe, and N and has a magnetic phase at least a portion of which has either a Th ₂ Zn ₁₇ type or a Th ₂ Ni ₁₇ type crystal structure.

High-performance rare earth magnets that have been put to practical use include Sm-Co rare earth magnets and Nd-Fe-B rare earth magnets, but in recent years, other rare earth magnets have been considered.

For example, rare earth magnets containing Sm, Fe, and N (hereinafter sometimes referred to as "Sm-Fe-N rare earth magnets") are being considered. Sm-Fe-N rare earth magnets are manufactured using, for example, magnetic powder containing Sm, Fe, and N (hereinafter sometimes referred to as "SmFeN powder").

SmFeN powder has a magnetic phase having either a Th ₂ Zn ₁₇ type or a Th ₂ Ni ₁₇ type crystal structure. This magnetic phase is thought to be a solid solution of N interstitially dissolved in Sm-Fe crystals. Therefore, SmFeN powder is easily decomposed by N being dissociated by heat. For this reason, Sm-Fe-N rare earth magnets are often manufactured by molding SmFeN powder with resin and/or rubber.

Other methods for manufacturing Sm-Fe-N rare earth magnets include, for example, the method disclosed in Patent Document 1. This method involves mixing SmFeN powder with a powder containing metallic zinc (hereinafter sometimes referred to as "metallic zinc powder"), compacting the mixed powder in a magnetic field, and sintering (including liquid phase sintering) the magnetically compacted body.

In addition, methods for producing SmFeN powder are disclosed, for example, in Patent Documents 2 and 3.

International Publication No. 2015/199096 JP 2017-117937 A JP 2020-102606 A

Methods for sintering magnetically compacted bodies can be broadly divided into pressureless sintering and pressure sintering. In either sintering method, a high-density rare earth magnet (sintered body) is obtained by sintering the magnetically compacted body. In the pressureless sintering method, no pressure is applied to the magnetically compacted body during sintering, so to obtain a high-density sintered body, it is common to sinter the magnetically compacted body at a high temperature of 900°C or more for a long period of time of 6 hours or more. On the other hand, in the pressure sintering method, pressure is applied to the magnetically compacted body during sintering, so it is common to obtain a high-density sintered body even if the magnetically compacted body is sintered at a low temperature of 600-800°C for a short period of time of 0.1-5 hours.

When sintering a magnetic compact of a mixture of SmFeN powder and metal zinc powder, pressure sintering is used to avoid thermal decomposition of the SmFeN powder, but the sintering temperature is lower and shorter than the sintering temperature of normal pressure sintering. Sintering is possible at such a low temperature and in such a short time because the zinc component in the metal zinc powder diffuses to the surface of the magnetic powder during sintering and sinters (solidifies). In this way, the metal zinc powder in the magnetic compact functions as a binder. In addition, the metal zinc powder in the magnetic compact also functions as a modifier, modifying the SmFeN powder, especially the αFe phase on the surface of the SmFeN powder particles, and absorbing oxygen in the SmFeN powder to improve the coercive force. Hereinafter, the powder used in the manufacture of Sm-Fe-N rare earth magnets and having both the functions of a binder and a modifier may be simply referred to as "modifier powder".

In both cases where the magnetic powder is molded using resin and/or rubber, etc., and where a mixed powder of the magnetic powder and modifier powder is pressure sintered, the magnetization of the molded body (rare earth magnet) decreases by the amount of the resin and modifier content that do not contribute to magnetization. On the other hand, compared to molding the magnetic powder using resin and/or rubber, etc., when a mixed powder of the magnetic powder and modifier powder is pressure sintered, a molded body (rare earth magnet) with high density is generally obtained, and as a result, it is easier to obtain high magnetization. However, when the magnetic powder is SmFeN powder, even if a mixed powder of the magnetic powder and modifier powder is pressure sintered, the magnetization is lower than predicted from the content ratio of the modifier, and the desired magnetization may not be obtained.

Based on these findings, the inventors have discovered that there is a need for a method for producing Sm-Fe-N rare earth magnets that can further improve magnetization compared to conventional methods.

This disclosure has been made to solve the above problems. In other words, the purpose of this disclosure is to provide a method for producing Sm-Fe-N rare earth magnets that can further improve magnetization compared to conventional methods.

The present inventors have conducted intensive research to achieve the above object, and have completed the method for producing a rare earth magnet according to the present disclosure. The method for producing a rare earth magnet according to the present disclosure includes the following aspects.
<1> A magnetic powder containing Sm, Fe, and N and having a magnetic phase at least a part of which has a crystal structure of either Th ₂ Zn ₁₇ type or Th ₂ Ni ₁₇ type is prepared;
Preparing a modifier powder containing at least one of metallic zinc and a zinc alloy;
mixing the magnetic powder and the modifier powder to obtain a mixed powder;
Compressing and molding the mixed powder in a magnetic field to obtain a magnetically molded body; and pressure-sintering the magnetically molded body to obtain a sintered body.
Including,
the magnetic powder includes a first particle group and a second particle group,
The particle size distribution _D50 of the first particle group is expressed in terms of _d1 μm, and the particle size distribution _D50 of the second particle group is expressed in terms of _d2 μm;
The _d1 and _d2 satisfy the relationship of 0.350≦ _d2 / _d1 ≦0.500, and
the ratio of the total volume of the first particle group to the total volume of the second particle group (total volume of the first particle group:total volume of the second particle group) is in the range of 9:1 to 4:1;
A method for manufacturing rare earth magnets.
<2> The method for producing a rare earth magnet according to item <1>, wherein _d1 is 3.0 to 3.7 μm and _d2 is 1.4 to 1.8 μm.
<3> The method for producing a rare earth magnet according to <1> or <2>, wherein the modifier powder has a _D50 of 0.1 to 12.0 μm, and the content of zinc in the modifier powder is 1 to 30 mass% with respect to the mixed powder.
<4> The method for producing a rare earth magnet according to any one of <1> to <3>, wherein the mixed powder is compression-molded at a pressure of 10 to 1500 MPa.
<5> The method for producing a rare earth magnet according to any one of <1> to <4>, wherein the magnetic compact is pressure-sintered at a pressure of 100 to 2000 MPa and a temperature of 300 to 430°C for 1 to 30 minutes.
<6> forming a modification-suppressing coating on the particle surfaces of the second particle group before the pressure sintering; and
heat-treating the sintered body to promote modification of the particle surfaces of the first particle group;
The method for producing a rare earth magnet according to any one of items <1> to <5>, further comprising:
<7> The method for producing a rare earth magnet according to <6>, wherein the modification-suppressing coating contains phosphoric acid.
<8> The method for producing a rare earth magnet according to <6> or <7>, wherein the sintered body is heat-treated at 350 to 410° C.
<9> A sintered rare earth magnet comprising: a magnetic powder containing Sm, Fe, and N, at least a portion of which has a magnetic phase having a crystal structure of either _{Th2Zn17 type} or _{Th2Ni17 type} ; and a zinc component,
the magnetic powder includes a first particle group and a second particle group,
The particle size distribution _D50 of the first particle group is expressed in terms of _d1 μm, and the particle size distribution _D50 of the second particle group is expressed in terms of _d2 μm;
the _d1 and the _d2 satisfy the relationship of 0.350≦ _d2 / _d1 ≦0.500, and the ratio of the total volume of the first particle group to the total volume of the second particle group (the total volume of the first particle group: the total volume of the second particle group) is in the range of 9:1 to 4:1.
Rare earth magnets.

According to the manufacturing method disclosed herein, the density of the sintered body (rare earth magnet) can be improved by setting the ratio of the particle size of the second particles to the particle size of the first particle group and the ratio of the total volume of the first particle group to the total volume of the second particle group within a predetermined range. As a result, it is possible to provide a manufacturing method for a rare earth magnet that can further improve magnetization compared to conventional methods.

FIG. 1 is a schematic diagram showing an example of the structure of a rare earth magnet obtained by the manufacturing method of the present disclosure. FIG. 2 is a schematic diagram showing an example of the structure of a rare earth magnet obtained by a conventional manufacturing method. FIG. 3 is a schematic diagram showing another example of the structure of a rare earth magnet obtained by a conventional manufacturing method. FIG. 4 is a schematic diagram showing yet another example of the structure of a rare earth magnet obtained by a conventional manufacturing method. FIG. 5 is a graph showing the relationship between d ₂ /d ₁ and density. FIG. 6 is a graph showing the relationship between d ₂ /d ₁ and the residual magnetization Br. FIG. 7 shows an SEM image of the sample of Example 1. FIG. 8 shows an SEM image of the sample of Comparative Example 3. FIG. 9 shows an SEM image of the sample of Comparative Example 6. FIG. 10 is a graph showing the demagnetization curve of a compact of low coercivity powder and a compact of a mixed powder of low coercivity powder and high coercivity powder at high temperatures. FIG. 11 is a schematic diagram showing an example of the structure of a rare earth magnet obtained by forming a modification-suppressing coating on the surfaces of the particles of the second particle group, followed by pressure sintering and heat treatment. FIG. 12 is a schematic diagram showing an example of the structure of a rare earth magnet obtained by pressure sintering and heat treatment without forming a modification-suppressing coating on the surfaces of the particles of the second particle group.

Below, an embodiment of the manufacturing method for rare earth magnets disclosed herein (hereinafter, sometimes simply referred to as the "manufacturing method of the present disclosure") will be described in detail. Note that the embodiment described below does not limit the manufacturing method of the present disclosure.

Without being bound by theory, the reason why the manufacturing method disclosed herein can produce rare earth magnets with improved magnetization compared to conventional methods will be explained using drawings and in comparison with conventional methods for manufacturing rare earth magnets (hereinafter sometimes simply referred to as "conventional manufacturing methods").

Figure 1 is a schematic diagram showing an example of the structure of a rare earth magnet obtained by the manufacturing method of the present disclosure. Figure 2 is a schematic diagram showing an example of the structure of a rare earth magnet obtained by a conventional manufacturing method. Figure 3 is a schematic diagram showing another example of the structure of a rare earth magnet obtained by a conventional manufacturing method. Figure 4 is a schematic diagram showing yet another example of the structure of a rare earth magnet obtained by a conventional manufacturing method. Note that the arrows in Figures 1 to 4 indicate the direction of magnetic orientation.

As shown in FIG. 1, the rare earth magnet 100 obtained by the manufacturing method of the present disclosure has SmFeN powder particles 10 bound together by modifier 20. This is because, as described above, modifier 20 functions as a binder. The surface of the SmFeN powder particles 10 is coated with modifier phase 30.

As shown in FIG. 1, in the rare earth magnet 100 obtained by the manufacturing method of the present disclosure, the SmFeN powder particles 10 include a first particle group 11 having a large particle size and a second particle group 12 having a small particle size. The presence of the particles of the second particle group 12 between the particles of the first particle group 11 increases the density of the rare earth magnet 100, thereby improving the magnetization. For example, as shown in FIG. 2, in an example of a rare earth magnet 200 obtained by a conventional manufacturing method, the SmFeN powder particles 10 are essentially only the first particle group 11, so the density of the rare earth magnet 200 cannot be increased, and as a result, the magnetization does not improve.

Also, as shown in FIG. 1, the density of the rare earth magnet 100 obtained by the manufacturing method of the present disclosure can be increased when the ratio of the particle size of the second particle group 12 to the particle size of the first particle group 11 is within a predetermined range. For example, as shown in FIG. 3, in another example of a rare earth magnet 200 obtained by a conventional manufacturing method, the ratio of the particle size of the second particle group 12 to the particle size of the first particle group 11 is too large, so the particles of the first particle group 11 are spaced apart. Therefore, the density of the rare earth magnet 200 obtained by the conventional manufacturing method cannot be increased, and as a result, the magnetization does not improve.

In addition, in order to increase the density of the rare earth magnet 100 obtained by the manufacturing method of the present disclosure, not only must the ratio of the particle size of the second particle group 12 to the particle size of the first particle group 11 be within a predetermined range, but the ratio of the total volume of the first particle group 11 to the total volume of the second particle group 12 must also be within a predetermined range. This is because, when there are a certain number of particles of the second particle group 12 or more, the gaps between the particles of the first particle group 11 are sufficiently filled as shown in FIG. 1, whereas when there are an excessive number of particles of the second particle group 12, the gaps between the particles of the first particle group 11 are expanded as shown in FIG. 4. This expansion makes it impossible to increase the density of the rare earth magnet 200 obtained by the conventional manufacturing method, and as a result, the magnetization is not improved.

Furthermore, compared to large SmFeN powder particles such as the first particle group 11, small SmFeN powder particles such as the second particle group 12 have a smaller particle remanence magnetization σr. This is because the crystal structure of the particle surface is deteriorated, and small particles have a larger specific surface area compared to large particles, so small particles such as the second particle group 12 are more likely to have a deteriorated particle remanence magnetization σr. If such second particle group 12 is present in excess, it will lead to a decrease in the magnetization of the entire rare earth magnet.

For these reasons, the manufacturing method disclosed herein avoids the presence of an excess of particles of the second particle group 12, thereby avoiding a decrease in magnetization.

Without being bound by theory, as described above, the reason why both the particle size ratio and the total volume ratio of the particles of the first particle group 11 and the second particle group 12 must be within a predetermined range is believed to be due to the following. Compared to magnetic powders used in the manufacture of Nd-Fe-B rare earth magnets, the friction coefficient of SmFeN powder particles is very large. Therefore, when molding SmFeN powder, its fluidity is not good, and it is difficult to increase the filling rate of the molded body (rare earth magnet). It can be understood that magnetization increases as density increases because residual magnetization can be expressed by the following formula.
Residual magnetization = saturation magnetization x degree of orientation x (density/true density) x magnetic phase ratio

The constituent elements of the manufacturing method disclosed herein, which was completed based on the findings discussed above, are described below.

《Manufacturing method》
The manufacturing method of the present disclosure includes a magnetic powder preparation step, a modifier powder preparation step, a mixing step, a magnetic field compaction step, and a pressure sintering step. It also optionally includes a modification-suppressing film formation step and a heat treatment step. Each step will be described below.

<Magnetic powder preparation process>
A magnetic powder (SmFeN powder) is prepared. The magnetic powder (SmFeN powder) used in the manufacturing method of the present disclosure is not particularly limited as long as it contains Sm, Fe, and N and has a magnetic phase having at least a part of a crystal structure of either Th ₂ Zn ₁₇ type or Th ₂ Ni ₁₇ type. In addition to the above-mentioned structure, the crystal structure of the magnetic phase may be a phase having a crystal structure of TbCu ₇ type. Sm is samarium, Fe is iron, and N is nitrogen. Th is thorium, Zn is zinc, Ni is nickel, Tb is terbium, and Cu is copper.

The SmFeN powder may contain a magnetic phase represented by the composition formula (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj ) 17Nh} . The rare earth magnet obtained by the manufacturing method of the present disclosure (hereinafter sometimes referred to as the "result") exhibits magnetization due to the magnetic phase in the SmFeN powder. Note that i, j, and h are molar ratios.

The magnetic phase in the SmFeN powder may contain R within a range that does not impair the effects of the manufacturing method of the present disclosure and the magnetic properties of the product. Such a range is represented by i in the above composition formula. i may be, for example, 0 or more, 0.10 or more, or 0.20 or more, and may be 0.50 or less, 0.40 or less, or 0.30 or less. R is one or more selected from rare earth elements other than Sm and Zr. In this specification, rare earth elements are Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In addition, Zr is zirconium, Sc is scandium, Y is yttrium, La is lanthanum, Ce is cerium, Pr is praseodymium, Nd is neodymium, Pm is promethium, Sm is samarium, Eu is europium, Gd is gadolinium, Tb is terbium, Dy is dysprosium, Ho is holmium, Er is erbium, Tm is thulium, Yb is ytterbium, and Lu is lutetium.

In the case of (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj ) 17Nh} , typically, R is substituted at the position of Sm in _Sm2 (Fe _{(1-j) Coj} ) _17Nh , but _this is not limited thereto. For example, part of R may be interstitially disposed in _Sm2 (Fe _{(1-j) Coj ) 17Nh} .

The magnetic phase in the SmFeN powder may contain Co within a range that does not impair the effects of the manufacturing method of the present disclosure or the magnetic properties of the resulting product. Such a range is represented by j in the above composition formula. j may be 0 or more, 0.10 or more, or 0.20 or more, and may be 0.52 or less, 0.40 or less, or 0.30 or less.

In the case of (Sm _{(1-i) Ri} ) ₂ (Fe _{( 1-j) Coj} ) _17Nh , typically, Co is substituted at the Fe position _of (Sm _{(1-i) Ri ) 2Fe17Nh ,} but this is not limited thereto. For example, part of Co may be disposed in an interstitial manner in (Sm _{(1-i) Ri} ) _{2Fe17Nh .}

The magnetic phase in the SmFeN powder has crystal grains represented by (Sm _(1-i) R _i ) ₂ (Fe _(1-j) Co _j ) ₁₇ , and N exists in an interstitial form, which contributes to the development and improvement of magnetic properties.

With respect to (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) C0j} ) _17Nh , h can be 1.5 to 4.5, but is typically (Sm _{( 1-i) Ri} ) ₂ (Fe _{(1-j) C0j} ) _17N3 . h may be 1.8 or more _, 2.0 or more, or 2.5 or more, and may be 4.2 or less, 4.0 or less, or 3.5 or less. _The content of (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj} ) _17N3 relative to the entire (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj} ) _17Nh is preferably ₇₀ mass% or more, more preferably 80 mass% or more, and even more preferably 90 mass%. On the other hand, all of (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj} ) _17Nh does not have to be (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj ) 17N3 . The} content of (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj} ) _17N3 relative to the entire (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj} ) _17Nh may be ₉₈ mass% or less, 95 mass% or less, or 92 mass% or less.

In addition to the magnetic phase represented by (Sm _(1-i) R _i ) ₂ (Fe _(1-j) Co _j ) ₁₇ N _h , the SmFeN powder may contain oxygen, M ¹ , and unavoidable impurity elements to the extent that the effect of the manufacturing method of the present disclosure and the magnetic properties of the product are not substantially impaired. From the viewpoint of ensuring the magnetic properties of the product, the content of the magnetic phase represented by (Sm _(1-i) R _i ) ₂ (Fe _(1-j) Co _j ) ₁₇ N _h relative to the entire SmFeN powder may be 80 mass% or more, 85 mass% or more, or 90 mass% or more. On the other hand, there is no practical problem even if the content of the magnetic phase represented by (Sm _(1-i) R _i ) ₂ (Fe _(1-j) Co _j ) ₁₇ N _h relative to the entire SmFeN powder is not excessively high. Therefore, the content may be 97 mass % or less, 95 mass % or less, or 93 mass % or less. The remainder of the magnetic phase represented by (Sm _{(1-i) Ri} ) ₂ (Fe _{(1-j) Coj ) 17Nh} is the content of oxygen and ^M1 . In addition, a part of oxygen and ^M1 may be present in the magnetic phase in the form of interstitial and/or substitutional type.

The above-mentioned M ¹ may be one or more selected from Ga, Ti, Cr, Zn, Mn, V, Mo, W, Si, Re, Cu, Al, Ca, B, Ni, and C. An unavoidable impurity element refers to an impurity element whose inclusion cannot be avoided when manufacturing raw materials and/or magnetic powders, or whose avoidance would lead to a significant increase in manufacturing costs. These elements may be present in the above-mentioned magnetic phases in a substitutional and/or interstitial form, or in phases other than the above-mentioned magnetic phases. Alternatively, they may be present in the grain boundaries of these phases. Ga is gallium, Ti is titanium, Cr is chromium, Zn is zinc, Mn is manganese, V is vanadium, Mo is molybdenum, W is tungsten, Si is silicon, Re is rhenium, Cu is copper, Al is aluminum, Ca is calcium, B is boron, Ni is nickel, and C is carbon.

The SmFeN powder includes a first particle group and a second particle group. The particles of the first particle group have a large particle size, and the particles of the second particle group have a small particle size. The particle size of each of the first particle group and the second particle group can be represented by the particle size distribution D _50. The particle size distribution D ₅₀ of the first particle group is represented by d ₁ μm, and the particle size distribution D ₅₀ of the second particle group is represented by d ₂ μm. And, d ₁ and d ₂ satisfy the relationship of 0.350≦d ₂ /d ₁ ≦0.500. Since the above relationship is satisfied, it is clear that d ₂ <d ₁ , that is, the first particle group has a large particle size, and the second particle group has a small particle size.

When _d2 / _d1 is 0.350 or more, 0.360 or more, 0.370 or more, or 0.378 or more and 0.500 or less, 0.490 or less, 0.486 or less, 0.480 or less, 0.470 or less, or 0.467 or less, the particles of the second particle group are advantageously present between the particles of the first particle group, increasing the density of the molded body (rare earth magnet) and, as a result, improving magnetization.

As long as the above relationship is satisfied, the particle diameters of the first particle group and the second particle group are not particularly limited, but in order to easily satisfy the above relationship, it is preferable that _d1 and _d2 are each independently in the following ranges. _d1 is preferably 3.0 μm or more, 3.2 μm or more, or 3.4 μm or more, and may be 3.7 μm or less, 3.6 μm or less, or 3.5 μm or less. _d2 is preferably 1.4 μm or more or 1.5 μm or more, and preferably 1.8 μm or less, 1.7 μm or less, or 1.6 μm or less.

The ratio of the total volume of the first particle group to the total volume of the second particle group, i.e., (total volume of the first particle group):(total volume of the second particle group), must be in the range of 9:1 to 4:1. A ratio of (total volume of the first particle group):(total volume of the second particle group) of 9:1 means, for example, that the total volume of the first particle group is 90% and the total volume of the second particle group is 10% of the total volume of the SmFeN powder. A ratio of (total volume of the first particle group):(total volume of the second particle group) of 4:1 means, for example, that the total volume of the first particle group is 80% and the total volume of the second particle group is 20% of the total volume of the SmFeN powder.

If the ratio of (total volume of the first particle group):(total volume of the second particle group) is 9:1 or the total volume of the second particle group is greater than this, the particles of the second particle group will be advantageously present between the particles of the first particle group, increasing the density of the rare earth magnet and, as a result, improving magnetization. From this perspective, it is preferable that the ratio of (total volume of the first particle group):(total volume of the second particle group) is 8.8:1.2 or more, or 8.6:1.4 or more.

If there is an excess of the second particle group, the gaps between the particles of the first particle group will be warped and enlarged. To avoid this, the ratio of (total volume of the first particle group): (total volume of the second particle group) must be 4:1 or the total volume of the second particle group must be less than that. In addition, small-sized SmFeN powder particles such as the second particle group have a small particle residual magnetization σr, so if there is an excess of the second particle group, the magnetization of the entire rare earth magnet will decrease. For this reason, by avoiding the expansion of the gaps between the particles of the first particle group, the decrease in density of the rare earth magnet is suppressed, and the magnetization of the rare earth magnet is improved by reducing the number of second particle groups with small σr. From these points of view, it is preferable that the ratio of (total volume of the first particle group): (total volume of the second particle group) is 8.2:1.8 or less or 8.4:1.6 or less.

The magnetic powder containing the first particle group and the second particle group is typically prepared by classifying the SmFeN powder obtained by the manufacturing method described below into the first particle group and the second particle group, and then mixing them again. There are no particular limitations on the classification and mixing method, and any well-known method may be used. Examples of classification methods include sieve classification and air classification, and these may be combined. Examples of mixing methods include mixing using an agitator mixer and a V-type mixer, and these may be combined.

The _D50 of the SmFeN powder is calculated from the particle size distribution of the SmFeN powder, which is measured (investigated) by the following method. In this specification, unless otherwise specified, descriptions regarding the particle size (particle size) of the SmFeN powder are based on the following measurement method (investigation method). Note that _D50 means the median diameter.

A sample was prepared by embedding SmFeN powder in resin, and the surface of the sample was polished and observed under an optical microscope. A straight line was then drawn on the optical microscope image, and the length of the line segment where the line was separated by SmFeN particles (bright field) was measured, and the particle size distribution of the SmFeN powder was obtained from the frequency distribution of the line segment lengths. The particle size distribution obtained by this method is approximately equal to the particle size distribution obtained by the intersection method or the dry laser diffraction/scattering method.

Although fine particles are present in the SmFeN powder for reasons of manufacturing convenience, etc., as long as _d1 and _d2 satisfy the above-mentioned relationship, there is no particular limitation on the ratio of magnetic particles (fine particles) having a particle size of 1.0 μm or less in the SmFeN powder. The ratio of magnetic particles (fine particles) having a particle size of 1.0 μm or less in the SmFeN powder is preferably as low as possible from the viewpoint of ensuring the mechanical strength of the compact (rare earth magnet). The ratio of fine particles to the total number of magnetic particles in the SmFeN powder is preferably 10.0% or less, 8.0% or less, 6.0% or less, or 4.0% or less. From the viewpoint of manufacturing convenience of the SmFeN powder, fine particles do not have to be completely absent, and there is no practical problem even if the lower limit of the ratio of fine particles is 1.0%, 2.0%, or 3.0%.

In the manufacturing method of the present disclosure, the modifier powder described below is mixed with the SmFeN powder. The oxygen in the SmFeN powder is absorbed by the metal zinc or zinc alloy powder in the modifier powder, thereby improving the magnetic properties of the compact, particularly the coercive force. The oxygen content in the SmFeN powder may be determined in consideration of the amount of oxygen absorbed by the modifier powder in the manufacturing process. The oxygen content of the SmFeN powder is preferably low relative to the entire SmFeN powder. The oxygen content of the SmFeN powder is preferably 2.0 mass% or less, more preferably 1.5 mass% or less, and even more preferably 1.0 mass% or less relative to the entire SmFeN powder. On the other hand, extremely reducing the oxygen content in the SmFeN powder leads to an increase in manufacturing costs. For this reason, the oxygen content of the SmFeN powder may be 0.1 mass% or more, 0.2 mass% or more, or 0.3 mass% or more, based on the entire SmFeN powder.

As long as the above-described conditions are satisfied, there is no particular restriction on the method of producing the SmFeN powder, and a commercially available product may be used. Examples of methods for producing SmFeN powder include a method in which Sm-Fe powder is produced from samarium oxide and iron powder by a reduction diffusion method, and then heat-treated at 600°C or less in an atmosphere of a mixed gas of nitrogen and hydrogen, nitrogen gas, ammonia gas, or the like to obtain Sm-Fe-N powder. Alternatively, examples include a method in which an Sm-Fe alloy is produced by a melting method, the alloy is coarsely crushed to obtain coarsely crushed particles, which are then nitrided and further crushed to the desired particle size. For example, a dry jet mill, a dry ball mill, a wet ball mill, or a wet bead mill can be used for the crushing. These may also be used in combination.

In addition to the above-mentioned manufacturing method, SmFeN powder can be obtained by a manufacturing method including a pretreatment step of heat-treating an oxide containing Sm and Fe in a reducing gas-containing atmosphere to obtain a partial oxide, a reduction step of heat-treating the partial oxide in the presence of a reducing agent to obtain alloy particles, and a nitriding step of heat-treating the alloy particles in a nitrogen or ammonia-containing atmosphere at a first temperature of 400°C to 470°C, and then at a second temperature of 480°C to 610°C to obtain nitrides. In particular, with alloy particles with a large particle size, such as alloy particles containing La, nitriding may not progress sufficiently to the inside of the oxide particles. However, when nitriding is performed at two temperatures, the inside of the oxide particles is also sufficiently nitrided, and anisotropic SmFeN powder with a narrow particle size distribution and high remanent magnetization can be obtained.

[Oxide preparation process]
The oxide containing Sm and Fe used in the pretreatment step described below may be prepared, for example, by mixing Sm oxide and Fe oxide. However, it is preferable to produce the oxide containing Sm and Fe by a step of mixing a solution containing Sm and Fe with a precipitant to obtain a precipitate containing Sm and Fe (precipitation step), and a step of calcining the precipitate to obtain an oxide containing Sm and Fe (oxidation step).

[Precipitation step]
In the precipitation step, a Sm raw material and an Fe raw material are dissolved in a strongly acidic solution to prepare a solution containing Sm and Fe. When Sm ₂ Fe ₁₇ N ₃ is obtained as the main phase, the molar ratio of Sm and Fe (Sm:Fe) is preferably 1.5:17 to 3.0:17, more preferably 2.0:17 to 2.5:17. Raw materials such as La, W, Co, Ti, Sc, Y, Pr, Nd, Pm, Gd, Tb, Dy, Ho, Er, Tm, and/or Lu may be added to the above-mentioned solution. In terms of residual magnetic flux density, it is preferable to contain La. In terms of coercive force and squareness ratio, it is preferable to contain W. In terms of temperature characteristics, it is preferable to contain Co and/or Ti.

The Sm raw material and the Fe raw material are not limited as long as they can be dissolved in a strong acidic solution. For example, in terms of ease of availability, the Sm raw material may be samarium oxide, and the Fe raw material may be FeSO _4. The concentration of the solution containing Sm and Fe may be appropriately adjusted within a range in which the Sm raw material and the Fe raw material are substantially dissolved in the acidic solution. In terms of solubility, the acidic solution may be sulfuric acid.

By reacting a solution containing Sm and Fe with a precipitant, an insoluble precipitate containing Sm and Fe is obtained. Here, the solution containing Sm and Fe may be a solution containing Sm and Fe when reacted with the precipitant. For example, raw materials containing Sm and Fe may be prepared as separate solutions, and each solution may be dropped to react with the precipitant. Even when preparing them as separate solutions, the raw materials are appropriately adjusted to a range in which each raw material is substantially soluble in the acidic solution. The precipitant is not limited as long as it is an alkaline solution that reacts with the solution containing Sm and Fe to obtain a precipitate, and examples of the precipitant include ammonia water and caustic soda, with caustic soda being preferred.

The precipitation reaction is preferably carried out by dropping the solution containing Sm and Fe and the precipitant into a solvent such as water, because the properties of the precipitate particles can be easily adjusted. By appropriately controlling the supply rate of the solution containing Sm and Fe and the precipitant, the reaction temperature, the concentration of the reaction solution, the pH during the reaction, etc., a precipitate with a uniform distribution of the constituent elements, a narrow particle size distribution, and a regular powder shape can be obtained. By using such a precipitate, the magnetic properties of the final product, SmFeN powder, are improved. The reaction temperature can be set to 0°C or higher and 50°C or lower, and preferably 35°C or higher and 45°C or lower. The reaction solution concentration is preferably 0.65 mol/L or higher and 0.85 mol/L or lower, and more preferably 0.7 mol/L or higher and 0.85 mol/L or lower, in terms of the total concentration of metal ions. The reaction pH is preferably 5 to 9, and more preferably 6.5 to 8.

The solution containing Sm and Fe preferably further contains one or more metals selected from the group consisting of La, W, Co, and Ti in terms of magnetic properties. For example, it is preferable to contain La in terms of residual magnetic flux density, it is preferable to contain W in terms of coercive force and squareness ratio, and it is preferable to contain Co and/or Ti in terms of temperature properties. The La raw material is not limited as long as it can be dissolved in a strongly acidic solution, and examples of the La raw material include La ₂ O ₃ and LaCl ₃ in terms of ease of availability. Together with the Sm raw material and the Fe raw material, the La raw material, the W raw material, the Co raw material, and the Ti raw material are appropriately adjusted within a range in which they are substantially dissolved in the acidic solution, and the acidic solution includes sulfuric acid in terms of solubility. The W raw material includes ammonium tungstate, the Co raw material includes cobalt sulfate, and the titanium raw material includes titania sulfate.

When the solution containing Sm and Fe further contains one or more metals selected from the group consisting of La, W, Co, and Ti, an insoluble precipitate containing Sm, Fe, and one or more selected from the group consisting of La, W, Co, and Ti is obtained. Here, the solution only needs to contain one or more selected from the group consisting of La, W, Co, and Ti when reacted with the precipitant. For example, each raw material may be prepared as a separate solution, and each solution may be dropped to react with the precipitant, or the solution may be prepared together with the solution containing Sm and Fe.

The powder obtained in the precipitation process largely determines the powder particle size, powder shape, and particle size distribution of the final SmFeN powder. When the particle size of the obtained powder is measured using a laser diffraction type wet particle size distribution analyzer, it is preferable that the size and distribution of all the powder is approximately in the range of 0.05 μm to 20 μm, preferably 0.1 μm to 10 μm.

After separating the precipitate, it is preferable to desolvate the separated material in order to prevent the precipitate from redissolving in the remaining solvent during the heat treatment in the subsequent oxidation step, causing the precipitate to aggregate and change in particle size distribution, powder particle size, etc. when the solvent evaporates. Specific examples of methods for desolvation include drying in an oven at 70°C to 200°C for 5 hours to 12 hours when water is used as the solvent.

The precipitation step may be followed by a step of separating and washing the resulting precipitate. The washing step is appropriately performed until the conductivity of the supernatant solution becomes 5 mS/ ^m2 or less. As a step of separating the precipitate, for example, a solvent (preferably water) may be added to the resulting precipitate and mixed, followed by filtration, decantation, or the like.

[Oxidation process]
The oxidation step is a step of obtaining an oxide containing Sm and Fe by firing the precipitate formed in the precipitation step. For example, the precipitate can be converted into an oxide by heat treatment. When the precipitate is heat treated, it needs to be performed in the presence of oxygen, for example, in an air atmosphere. In addition, since it needs to be performed in the presence of oxygen, it is preferable that the non-metallic part in the precipitate contains oxygen atoms.

The heat treatment temperature in the oxidation step (hereinafter sometimes referred to as "oxidation temperature") is not particularly limited, but is preferably 700°C or higher and 1300°C or lower, and more preferably 900°C or higher and 1200°C or lower. If the temperature is lower than 700°C, the oxidation will be insufficient, and if the temperature exceeds 1300°C, the desired shape, average particle size, and particle size distribution of the SmFeN powder will tend not to be obtained. The heat treatment time is also not particularly limited, but is preferably 1 hour or higher and 3 hours or lower.

The resulting oxide particles have sufficient microscopic mixing of Sm and Fe within the oxide particles, and reflect the shape and particle size distribution of the precipitates.

[Pretreatment process]
The pretreatment step is a step in which the oxide containing Sm and Fe is heat-treated in an atmosphere containing a reducing gas to obtain a partial oxide in which a part of the oxide is reduced.

Here, partial oxide refers to an oxide in which a part of an oxide has been reduced. There are no particular limitations on the oxygen concentration of the partial oxide, but it is preferably 10% by mass or less, and more preferably 8% by mass or less. If it exceeds 10% by mass, the reduction heat generated by Ca in the reduction process increases, and the firing temperature increases, tending to produce particles that have undergone abnormal particle growth. Here, the oxygen concentration of the partial oxide can be measured by non-dispersive infrared absorption (ND-IR).

The reducing gas is appropriately selected from hydrocarbon gases such as hydrogen (H ₂ ), carbon monoxide (CO), and methane (CH ₄ ), but hydrogen gas is preferred from the viewpoint of cost, and the flow rate of the gas is appropriately adjusted within a range in which the oxide does not scatter. The heat treatment temperature in the pretreatment step (hereinafter, pretreatment temperature) is preferably 300° C. or more and 950° C. or less, and the lower limit is more preferably 400° C. or more, and even more preferably 750° C. or more. The upper limit is more preferably less than 900° C. When the pretreatment temperature is 300° C. or more, the reduction of the oxide containing Sm and Fe proceeds efficiently. Furthermore, when the pretreatment temperature is 950° C. or less, the particle growth and segregation of the oxide particles are suppressed, and the desired particle size can be maintained. The heat treatment time is not particularly limited, but can be 1 hour or more and 50 hours or less. Furthermore, when hydrogen is used as the reducing gas, it is preferable to adjust the thickness of the oxide layer used to 20 mm or less, and further adjust the dew point in the reaction furnace to −10° C. or less.

[Reduction process]
The reduction step is a step of obtaining alloy particles by heat treating the partial oxide in the presence of a reducing agent, for example, by contacting the partial oxide with calcium melt or calcium vapor. From the viewpoint of magnetic properties, the heat treatment temperature is preferably 920° C. or more and 1200° C. or less, more preferably 950° C. or more and 1150° C. or less, and even more preferably 980° C. or more and 1100° C. or less.

The reducing agent, metallic calcium, is used in granular or powder form, with the particle size preferably being 10 mm or less. This makes it possible to more effectively suppress agglomeration during the reduction reaction. In addition, metallic calcium is preferably added in an amount 1.1 to 3.0 times the reaction equivalent (the stoichiometric amount required to reduce the rare earth oxide, and if the Fe component is in the form of an oxide, includes the amount required to reduce it), and more preferably 1.5 to 2.5 times.

In the reduction step, a disintegration promoter can be used as necessary together with the reducing agent, calcium metal. This disintegration promoter is used as appropriate to promote disintegration and granulation of the product in the post-treatment step described below, and examples of such disintegration promoters include alkaline earth metal salts such as calcium chloride and alkaline earth oxides such as calcium oxide. These disintegration promoters are used in a ratio of 1% by mass to 30% by mass, preferably 5% by mass to 30% by mass, per samarium oxide.

[Nitriding process]
The nitriding step is a step of obtaining anisotropic magnetic particles by heat-treating the alloy particles obtained in the reduction step at a first temperature of 400°C to 470°C in a nitrogen or ammonia-containing atmosphere, and then heat-treating the alloy particles at a second temperature of 480°C to 610°C in a nitriding process. Since the particulate precipitate obtained in the precipitation step is used, the alloy particles in the form of porous aggregates are obtained in the reduction step. This allows the alloy particles to be heat-treated and nitrided immediately in a nitrogen atmosphere without pulverization, so that the nitriding can be performed uniformly. If the alloy particles are heat-treated at a high second temperature without nitriding at the first temperature, the nitriding process will proceed rapidly, causing abnormal heat generation, which may cause SmFeN to decompose and greatly reduce the magnetic properties. In addition, the atmosphere in the nitriding step is preferably a substantially nitrogen-containing atmosphere, since the nitriding process can slow down the progress of nitriding. The term "substantially" used here is used in consideration of the fact that elements other than nitrogen are inevitably included due to the inclusion of impurities, etc., and for example, the ratio of nitrogen in the atmosphere is 95% or more, preferably 97% or more, and more preferably 99% or more.

The first temperature in the nitriding step is 400°C or higher and 470°C or lower, but preferably 410°C or higher and 450°C or lower. Below 400°C, nitriding proceeds very slowly, while above 470°C, over-nitriding or decomposition is likely to occur due to heat generation. The heat treatment time at the first temperature is not particularly limited, but is preferably 1 hour or higher and 40 hours or lower, and more preferably 20 hours or lower. Below 1 hour, nitriding may not proceed sufficiently, and above 40 hours, productivity decreases.

The second temperature is 480°C or more and 610°C or less, but preferably 500°C or more and 550°C or less. If the temperature is less than 480°C, nitriding may not proceed sufficiently if the particles are large, and if the temperature exceeds 610°C, over-nitriding or decomposition is likely to occur. The heat treatment time at the second temperature is preferably 15 minutes to 5 hours, and more preferably 30 minutes to 2 hours. If the temperature is less than 15 minutes, nitriding may not proceed sufficiently, and if the temperature exceeds 5 hours, productivity decreases.

The heat treatment at the first temperature and the heat treatment at the second temperature may be performed consecutively, and a heat treatment at a temperature lower than the second temperature may be included between these heat treatments, but from the viewpoint of productivity, it is preferable to perform them consecutively.

[Post-processing process]
The product obtained after the nitriding step contains, in addition to the magnetic particles, by-produced CaO, unreacted metallic calcium, etc., and these may be in the form of a composite sintered mass. The product obtained after the nitriding step can be put into cooling water to separate the CaO and metallic calcium as a calcium hydroxide (Ca(OH) ₂ ) suspension. Furthermore, the remaining calcium hydroxide may be sufficiently removed by washing the magnetic powder with acetic acid or the like. When the product is put into water, the composite sintered mass reaction product is disintegrated, i.e., pulverized, by the oxidation of metallic calcium by water and the hydration reaction of the by-produced CaO.

[Alkaline treatment step]
The product obtained after the nitriding step may be put into an alkaline solution. Examples of the alkaline solution used in the alkaline treatment step include an aqueous calcium hydroxide solution, an aqueous sodium hydroxide solution, and an aqueous ammonia solution. Among these, an aqueous calcium hydroxide solution and an aqueous sodium hydroxide solution are preferred in terms of wastewater treatment and high pH. The alkaline treatment of the product leaves an Sm-rich layer containing a certain amount of oxygen, which functions as a protective layer, thereby suppressing an increase in oxygen concentration due to the alkaline treatment.

The pH of the alkaline solution used in the alkaline treatment step is not particularly limited, but is preferably 9 or higher, and more preferably 10 or higher. If the pH is less than 9, the reaction rate when calcium hydroxide is produced is fast and heat is generated, so the oxygen concentration of the final SmFeN powder tends to be high.

In the alkaline treatment step, the moisture content of the SmFeN powder obtained after treatment with the alkaline solution can be reduced by a method such as decantation if necessary.

[Acid treatment step]
The alkaline treatment step may be followed by an acid treatment step in which the SmFeN powder is treated with an acid. In the acid treatment step, at least a part of the Sm-rich layer is removed to reduce the oxygen concentration in the entire SmFeN powder. In addition, in the manufacturing method according to the embodiment of the present invention, since no pulverization or the like is performed, the average particle size of the SmFeN powder is small, the particle size distribution is narrow, and no fine powder generated by pulverization or the like is included, so that it is possible to suppress an increase in the oxygen concentration.

The acid used in the acid treatment step is not particularly limited, and examples include hydrogen chloride, nitric acid, sulfuric acid, and acetic acid. Among these, hydrogen chloride and nitric acid are preferred because they do not leave behind any impurities.

The amount of acid used in the acid treatment step is preferably 3.5 parts by mass or more and 13.5 parts by mass or less, more preferably 4 parts by mass or more and 10 parts by mass or less, per 100 parts by mass of SmFeN powder. If the amount is less than 3.5 parts by mass, oxides remain on the surface of the SmFeN powder, and the oxygen concentration becomes high. If the amount of acid exceeds 13.5 parts by mass, reoxidation is likely to occur when exposed to the atmosphere, and the SmFeN powder is dissolved, so the cost tends to be high. By setting the amount of acid to 3.5 parts by mass or more and 13.5 parts by mass or less per 100 parts by mass of SmFeN powder, it is possible to make the Sm-rich layer oxidized to a degree that reoxidation is unlikely to occur when exposed to the atmosphere after acid treatment cover the surface of the SmFeN powder, so that SmFeN powder with a low oxygen concentration, small average particle size, and narrow particle size distribution can be obtained.

In the acid treatment step, the moisture content of the SmFeN powder obtained after the acid treatment can be reduced by a method such as decantation if necessary.

[Dehydration process]
It is preferable to include a dehydration step after the acid treatment step. The dehydration step reduces the moisture content in the solid content before vacuum drying, and suppresses the progress of oxidation during drying that occurs when the solid content before vacuum drying contains more moisture. Here, the dehydration step means a process in which pressure or centrifugal force is applied to reduce the moisture content of the solid content after treatment compared to the solid content before treatment, and does not include simple decantation, filtration, or drying. The dehydration method is not particularly limited, but examples include squeezing, centrifugation, and the like.

The amount of moisture contained in the SmFeN powder after the dehydration process is not particularly limited, but in order to suppress the progression of oxidation, it is preferably 13% by mass or less, and more preferably 10% by mass or less.

The SmFeN powder obtained by acid treatment, or the SmFeN powder obtained by dehydration after acid treatment, is preferably vacuum dried. The drying temperature is not particularly limited, but is preferably 70°C or higher, and more preferably 75°C or higher. The drying time is also not particularly limited, but is preferably 1 hour or more, and more preferably 3 hours or more.

<Modifier powder preparation process>
A modifier powder is prepared. The modifier powder used in the manufacturing method of the present disclosure contains at least one of metallic zinc and a zinc alloy. Metallic zinc means unalloyed zinc. The zinc component in the modifier powder bonds and modifies the particles of the SmFeN powder.

In the SmFeN powder particles, an Fe-Zn alloy phase is formed on the surface. The surface of the SmFeN powder particles has a portion with an incomplete crystal structure such as Th ₂ Zn ₁₇ type and/or Th ₂ Ni ₁₇ type, and an α-Fe phase is present in that portion, which causes a decrease in coercive force. This α-Fe phase forms an Fe-Zn alloy phase with metal zinc and/or the zinc component of the zinc alloy, suppressing a decrease in coercive force. That is, the Fe-Zn alloy phase acts as a modifier phase. Fe and Zn are mutually diffused between the SmFeN powder particles and the modifier powder particles, forming an Fe-Zn alloy phase. Therefore, the SmFeN powder particles can be firmly bound. That is, the modifier powder functions as a binder.

If the content of the zinc component in the modifier powder is 1% by mass or more relative to the mixed powder, a homogeneous Fe-Zn alloy phase (modifier phase) is formed, improving the coercive force and advantageously functioning as a binder. From this perspective, the content of metallic zinc in the modifier powder may be 3% by mass or more, 5% by mass or more, 10% by mass or more, 15% by mass or more, or 20% by mass or more relative to the mixed powder.

On the other hand, if the content of the zinc component in the modifier powder is 30% by mass or less relative to the mixed powder, the decrease in magnetization due to the use of the modifier powder can be suppressed. From this perspective, the content of the zinc component in the modifier powder may be 28% by mass or less, 26% by mass or less, 24% by mass or less, or 22% by mass or less relative to the mixed powder.

When the zinc alloy is represented by Zn-M ² , M ² may be selected as an element that, when alloyed with Zn (zinc), lowers the melting start temperature of the zinc alloy below the melting point of Zn and an inevitable impurity element. This improves sinterability in the pressure sintering process described later. Examples of M ² that lowers the melting point of Zn include elements that form a eutectic alloy with Zn and M ^2. Examples of such M ² include Sn, Mg, and Al, as well as combinations thereof. Sn is tin, Mg is magnesium, and Al is aluminum. Elements that do not inhibit the melting point lowering effect of these elements and the characteristics of the product can also be selected as M ^2. In addition, inevitable impurity elements refer to impurity elements that cannot be avoided, such as impurities contained in the raw materials of the modifier powder, or impurity elements that cannot be avoided, or that would lead to a significant increase in manufacturing costs if avoided.

In the zinc alloy represented by Zn- ^M2 , the ratio (molar ratio) of Zn and ^M2 may be appropriately determined so that the sintering temperature is appropriate. The ratio (molar ratio) of ^M2 to the entire zinc alloy may be, for example, 0.05 or more, 0.10 or more, or 0.20 or more, and may be 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.30 or less.

In addition to metallic zinc and/or zinc alloy, the modifier powder may optionally contain substances having binder and/or modifier functions and other functions, as long as the effects of the present invention are not impaired. Examples of other functions include the function of improving corrosion resistance.

The particle size of the modifier powder is not particularly limited, but is preferably smaller than the particle size of the SmFeN powder of the first particle group, and more preferably smaller than the particle size of the SmFeN powder of the second particle group. This makes it easier for the modifier powder particles to spread between the particles of the SmFeN powder. The particle size of the modifier powder may be, for example, 0.1 μm or more, 0.2 μm or more, 0.3 μm or more, or 0.4 μm or more in terms of D ₅₀ (median diameter), and may be 12.0 μm or less, 11.0 μm or less, 10.0 μm or less, 9.0 μm or less, 8.0 μm or less, 7.0 μm or less, 6.0 μm or less, 5.0 μm or less, 4.0 μm or less, 2.0 μm or less, 1.0 μm or less, or 0.5 μm or less. The particle diameter D ₅₀ (median diameter) of the modifier powder is measured, for example, by a dry laser diffraction/scattering method.

A low oxygen content in the modifier powder is preferable because it can absorb a large amount of oxygen in the SmFeN powder. From this perspective, the oxygen content of the modifier powder is preferably 5.0 mass% or less, more preferably 3.0 mass% or less, and even more preferably 1.0 mass% or less, relative to the entire modifier powder. On the other hand, extremely reducing the oxygen content of the modifier powder leads to an increase in manufacturing costs. For this reason, the oxygen content of the modifier powder may be 0.1 mass% or more, 0.2 mass% or more, or 0.3 mass% or more, relative to the entire modifier powder.

<Mixing process>
The SmFeN powder and the modifier powder are mixed to obtain a mixed powder. There is no particular limitation on the mixing method. Examples of the mixing method include a method using a mortar, a Muller wheel mixer, an agitator mixer, a mechanofusion, a V-type mixer, and a ball mill. These methods may be combined. The V-type mixer is a device that includes a container in which two cylindrical containers are connected in a V shape, and by rotating the container, the powder in the container is repeatedly aggregated and separated by gravity and centrifugal force, and mixed.

<Magnetic field forming process>
The mixed powder is compressed in a magnetic field to obtain a magnetically compacted body, which can impart orientation to the magnetically compacted body and anisotropy to the compact (rare earth magnet), improving remanence.

The magnetic field molding method may be a well-known method such as a method of compressing and molding the mixed powder using a mold around which a magnetic field generating device is installed. The molding pressure may be, for example, 10 MPa or more, 20 MPa or more, 30 MPa or more, 50 MPa or more, 100 MPa or more, or 150 MPa or more, and may be 1500 MPa or less, 1000 MPa or less, or 500 MPa or less. The time for applying the above-mentioned molding pressure may be, for example, 0.5 minutes or more, 1 minute or more, or 3 minutes or more, and may be 10 minutes or less, 7 minutes or less, or 5 minutes or less. The magnitude of the applied magnetic field may be, for example, 500 kA/m or more, 1000 kA/m or more, 1500 kA/m or more, or 1600 kA/m or more, and may be 20000 kA/m or less, 15000 kA/m or less, 10000 kA/m or less, 5000 kA/m or less, 3000 kA/m or less, or 2000 kA/m or less. Examples of methods for applying a magnetic field include a method of applying a static magnetic field using an electromagnet and a method of applying a pulse magnetic field using an alternating current. In addition, in order to suppress oxidation of the mixed powder, it is preferable that the magnetic field molding is performed in an inert gas atmosphere. The inert gas atmosphere includes a nitrogen gas atmosphere.

<Pressure sintering process>
The magnetic field compact is pressure-sintered to obtain a sintered body. The pressure-sintering method is not particularly limited, and any known method can be applied. As the pressure-sintering method, for example, a method is given in which a die having a cavity and a punch capable of sliding inside the cavity are prepared, the magnetic field compact is inserted inside the cavity, and the magnetic field compact is sintered while applying pressure to the magnetic field compact with the punch. In this method, typically, the die is heated using a high-frequency induction coil. Alternatively, a discharge plasma sintering (SPS) method may be used.

The pressure sintering conditions can be appropriately selected so that the magnetic field compact can be sintered (hereinafter sometimes referred to as "pressure sintered") while pressure is applied to the magnetic field compact.

If the sintering temperature is 300°C or higher, the Fe on the particle surface of the SmFeN powder and the metallic zinc of the modifier powder will slightly interdiffuse in the magnetic field compact, contributing to sintering. From this perspective, the sintering temperature may be, for example, 310°C or higher, 320°C or higher, 340°C or higher, or 350°C or higher. On the other hand, if the sintering temperature is 430°C or lower, the Fe on the particle surface of the SmFeN powder and the metallic zinc of the modifier powder will not excessively interdiffuse, and will not cause any problems in the heat treatment process described below or adversely affect the magnetic properties of the obtained sintered body. From these perspectives, the sintering temperature may be 420°C or lower, 410°C or lower, 400°C or lower, 390°C or lower, 380°C or lower, 370°C or lower, or 360°C or lower.

The sintering pressure may be appropriately selected so as to increase the density of the sintered body. The sintering pressure may typically be 100 MPa or more, 200 MPa or more, 400 MPa or more, 600 MPa or more, 800 MPa or more, or 1000 MPa or more, and may be 2000 MPa or less, 1800 MPa or less, 1600 MPa or less, 1500 MPa or less, 1300 MPa or less, or 1200 MPa or less.

The sintering time may be appropriately determined so that the Fe on the particle surface of the SmFeN powder and the modifier powder slightly interdiffuse. The sintering time does not include the time required to raise the temperature to the heat treatment temperature. The sintering time may be, for example, 1 minute or more, 2 minutes or more, or 3 minutes or more, and may be 30 minutes or less, 20 minutes or less, 10 minutes or less, or 5 minutes or less.

After the sintering time has elapsed, the sintered body is cooled to terminate the sintering. The faster the cooling rate, the more the oxidation of the sintered body can be suppressed. The cooling rate may be, for example, 0.5 to 200°C/sec.

As for the sintering atmosphere, an inert gas atmosphere such as an argon gas atmosphere is preferable to suppress oxidation of the magnetic compact and the sintered body. The inert gas atmosphere includes a nitrogen gas atmosphere.

As described above, the Fe on the surface of the SmFeN powder particles and the modifier powder undergo slight interdiffusion during pressure sintering, but optionally, some of this slight interdiffusion can be allowed to progress to allow the modification to proceed. In this case, a modification-suppressing coating formation process and a heat treatment process are performed. The modification-suppressing coating formation process and the heat treatment process are described below.

<Modification suppression coating formation process>
Prior to pressure sintering, a modification-inhibiting coating is formed on the particle surfaces of the second particle group in advance. This makes it possible to inhibit modification of the particle surfaces of the second particle group. The modification-inhibiting coating may be formed before pressure sintering, typically before mixing the SmFeN powder and the modifier powder.

The reason for suppressing modification of the particle surface of the second particle group is explained below.

By further heat treating the sintered body obtained by pressure sintering, the interdiffusion of Fe on the surface of the SmFeN powder particles and the modifier powder progresses, and the modification progresses, contributing to an improvement in the coercive force. Details of the heat treatment process will be described later.

The SmFeN powder used in the manufacturing method of the present disclosure includes a first particle group having a large particle size and a second particle group having a small particle size. This improves the density of the sintered body, and as a result, the magnetization improves. When the sintered body obtained in this manner is heat-treated, the particles of the second particle group are easily modified because of their large specific surface area, and some of the magnetic phase in the particles of the second particle group may also be modified. In that case, even if the density of the sintered body is improved, the magnetization may decrease somewhat. Therefore, it is preferable to form a modification-suppressing coating on the particle surfaces of the second particle group before pressure sintering to suppress the modification of the particle surfaces of the second particle group. This makes it possible to avoid some of the magnetic phase in the particles of the second particle group being modified, and as a result, it is possible to avoid some decrease in magnetization.

As described above, the first particle group and the second particle group are obtained by classifying SmFeN powder. In this case, since the second particle group has a higher coercive force than the first particle group, the second particle group does not need to be modified to the same extent as the first particle group. Therefore, it is advantageous to suppress modification of the particle surfaces of the second particle group.

In addition, when magnetic powders having high coercivity (hereinafter sometimes referred to as "high coercivity powders") and magnetic powders having low coercivity (hereinafter sometimes referred to as "low coercivity powders") coexist in a magnetic powder, the squareness of the compact made of such magnetic powders, especially at high temperatures, may decrease. This can be explained as follows using drawings. In this specification, unless otherwise specified, "high temperature" in relation to magnetic properties means 100 to 200°C, and squareness is evaluated at 10% demagnetization Hk.

Figure 10 is a graph showing the demagnetization curves of a compact made of low coercivity powder and a compact made of a mixed powder of low coercivity powder and high coercivity powder at high temperatures. It can be seen from Figure 10 that compared to the compact made of the mixed powder of low coercivity powder and high coercivity powder, the compact made of low coercivity powder has inferior coercivity but superior squareness.

As described above, the first particle group corresponds to a low coercive force powder, and the second particle group corresponds to a high coercive force powder. By forming a modification-inhibiting coating on the second particle group, it is possible to inhibit the second particle group from having an even higher coercive force. This makes it possible to inhibit the difference between the coercive force of the first particle group and the coercive force of the second particle group from increasing, and improves the squareness. As a result, by improving the density of the sintered body between the first particle group and the second particle group, it is possible to improve the squareness even when the magnetization is improved, which is even more preferable.

There are no particular limitations on the modification-inhibiting coating, so long as it can inhibit interdiffusion between the magnetic phase in the particles of the second particle group and the Fe on the particle surfaces of the second particle group, and the modifier powder, and does not adversely affect the magnetic properties of the rare earth magnet obtained by the manufacturing method of the present disclosure. Such a modification-inhibiting coating typically contains phosphoric acid, but is not limited to this.

When the modification-inhibiting coating is a coating containing phosphoric acid, the phosphoric acid content in the modification-inhibiting coating may be 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more, or may be 100% by mass, based on the entire modification-inhibiting coating. Also, when the modification-inhibiting coating is a coating containing phosphoric acid, its thickness may be 5 nm or more, 10 nm or more, 20 nm or more, 30 nm or more, 40 nm or more, or 50 nm or more, and may be 100 nm or less, 90 nm or less, 80 nm or less, 70 nm or less, or 60 nm or less.

There are no particular limitations on the method for forming a coating containing phosphoric acid on the second particle group, but examples include the following methods:

By subjecting the particles of the second particle group to a phosphate treatment, a passivation film having a P-O bond is formed on the particle surface of the second particle group. In the phosphate treatment process, a phosphate treatment agent is reacted with the particles of the second particle group. Examples of the phosphate treatment agent include inorganic phosphoric acids and organic phosphoric acids such as phosphate salts such as orthophosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, zinc phosphate, and calcium phosphate, hypophosphorous acid, hypophosphite salts, pyrophosphoric acid, and polyphosphoric acid. These phosphate sources are basically dissolved in water or an organic solvent such as an IPN, and the particles of the second particle group are placed in a phosphate bath to which a reaction accelerator such as nitrate ions and a crystal refiner such as V ions, Cr ions, and Mo ions have been added as necessary, to form a passivation film having a P-O bond on the particle surface of the second particle group.

<Heat treatment process>
In the case where a modification-inhibiting coating is formed on the particle surfaces of the second particle group before pressure sintering (hereinafter, simply referred to as "forming a modification-inhibiting coating on the particle surfaces of the second particle group"), the sintered body after pressure sintering is heat-treated to promote modification of the particle surfaces of the first particle group in the sintered body. This will be explained with reference to the drawings.

Figure 11 is a schematic diagram showing an example of the structure of a rare earth magnet obtained by forming a modification-inhibiting coating on the particle surfaces of the second particle group, and then performing pressure sintering and heat treatment in the manufacturing method disclosed herein. Figure 12 is a schematic diagram showing an example of the structure of a rare earth magnet obtained by not forming a modification-inhibiting coating on the particle surfaces of the second particle group, and performing pressure sintering and heat treatment. The structures of Figures 11 and 12 will be explained in comparison with the structure of Figure 1.

Compared to the structure in FIG. 1, the modified phase 30 on the particle surface of the first particle group 11 is slightly thicker in both the structures in FIG. 11 and FIG. 12. This is because, to obtain the rare earth magnet having the structure in FIG. 1, the sintered body is not heat-treated, whereas to obtain the rare earth magnet having the structure in FIG. 11 and FIG. 12, the sintered body is heat-treated, and the heat treatment causes the modification of the particle surface of the first particle group 11. Since the particles of the first particle group 11 have a relatively small specific surface area, the heat treatment does not cause excessive modification, and the modified phase 30 only becomes slightly thicker compared to the case where no heat treatment is performed.

Compared to the structure of FIG. 1, in the structure of FIG. 11, the thickness of the modified phase 30 on the particle surface of the second particle group 12 is almost the same, whereas in the structure of FIG. 12, the modified phase 30 on the particle surface of the second particle group 12 is thicker. This is because, to obtain a rare earth magnet having the structure of FIG. 11, a modification-suppressing coating is formed on the particle surface of the second particle group 12, whereas, to obtain a rare earth magnet having the structure of FIG. 12, a modification-suppressing coating is not formed on the particle surface of the second particle group 12. As a result, when obtaining a rare earth magnet having the structure of FIG. 11, the modification of the particle surface of the second particle group 12 hardly progresses during the heat treatment of the sintered body, whereas, when obtaining a rare earth magnet having the structure of FIG. 12, the modification of the particle surface of the second particle group 12 easily progresses during the heat treatment of the sintered body. In the structure of FIG. 11, the progress of the modification of the particle surface of the second particle group 12 is suppressed, and therefore the squareness is improved. Therefore, it is preferable to form a modification-suppressing coating on the particle surfaces of the second particle group 12.

Without being bound by theory, it is believed that during heat treatment of the sintered body after pressure sintering, the modification-inhibiting coating formed on the particle surfaces of the second particle group is decomposed into the elements that make up the modification-inhibiting coating, and these elements are present in the modification phase. From this, it is believed that the modification phase is a phase in which the elements originating from the above-mentioned modification-inhibiting coating are present in the Fe-Zn alloy phase.

The heat treatment conditions for the sintered body after pressure sintering may be appropriately determined so as to modify the surface of the SmFeN powder particles, particularly the surface of the particles of the first particle group. The heat treatment temperature may be, for example, 350°C or more, 360°C or more, 370°C or more, or 380°C or more, and 410°C or less, 400°C or less, or 390°C or less. The heat treatment time may be 6 hours or more, 12 hours or more, or 18 hours or more, and 48 hours or less, 42 hours or less, 36 hours or less, 30 hours or less, or 24 hours or less.

When the sintered body after pressure sintering is heat treated under the above-mentioned heat treatment conditions, the thickness of the modified phase on the particle surface of the first particle group is, for example, about 20 to 50 nm. This is because a modification-inhibiting coating is not formed on the particle surface of the first particle group. Furthermore, when the sintered body after pressure sintering is heat treated under the above-mentioned heat treatment conditions, the thickness of the modified phase on the particle surface of the second particle group is about 20 to 50 nm when no modification-inhibiting coating is formed, and about 1 to 20 nm when a modification-inhibiting coating is formed.

In order to suppress oxidation of the sintered body, it is preferable to heat treat the sintered body in a vacuum or in an inert gas atmosphere, including a nitrogen gas atmosphere. The heat treatment of the sintered body may be performed in the mold used for the pressure sintering, following the pressure sintering, but in that case, no pressure is applied to the sintered body during the heat treatment. The mold used for the pressure sintering is, for example, a die having a cavity. If the above-mentioned heat treatment conditions are satisfied, the normal magnetic phase decomposes to generate an α-Fe phase, and as a result of this generation, Fe and Zn do not diffuse excessively with each other. When the heat treatment is performed in a vacuum, the absolute pressure of the atmosphere may be 1×10 ⁻⁷ Pa or more, 1×10 ⁻⁶ Pa or more, or 1×10 ⁻⁵ Pa or more, and may be 1×10 ⁻² Pa or less, 1×10 ⁻³ Pa or less, or 1×10 ⁻⁴ Pa or less.

The rare earth magnets obtained using the manufacturing method disclosed above are described below.

Rare earth magnets
The rare earth magnet obtained by the manufacturing method of the present disclosure (hereinafter, sometimes referred to as the "rare earth magnet of the present disclosure") contains Sm, Fe, and N, and has a magnetic phase at least a part of which has either _{a Th2Zn17} type or a _{Th2Ni17 type} crystal structure. The composition of the magnetic phase is as described in "Magnetic powder preparation process".

The rare earth magnet of the present disclosure is obtained by using a mixed powder of SmFeN powder and a modifier powder containing at least one of metallic zinc and a zinc alloy. Therefore, the rare earth magnet of the present disclosure contains a zinc component derived from the modifier powder. As described above, a part of the particles of the SmFeN powder and a part of the zinc component of the modifier powder diffuse into each other to form an Fe-Zn alloy phase. In this specification, unless otherwise specified, the content of the "zinc component" means the content (content ratio) of Zn (zinc element). The zinc component of the rare earth magnet of the present disclosure is derived from the metallic zinc of the modifier powder, and the range of the zinc component content is preferably 1 to 30 mass%.

《Transform》
In addition to what has been described above, the manufacturing method of the present disclosure can be modified in various ways within the scope of the claims.

For example, some of the fine particles in the SmFeN powder may be removed in advance before the magnetic field compaction. There are no particular limitations on the fine particle removal operation (fine particle removal method). Examples of fine particle removal operations (fine particle removal methods) include a method using a cyclone (registered trademark) classifier, a method using a sieve, a method using a magnetic field, and a method using static electricity. Combinations of these methods are also possible. By removing the fine particles, the density of the compact (rare earth magnet) can be further increased, and the magnetization can be further increased.

The manufacturing method of the present disclosure will be explained in more detail below with reference to examples and comparative examples. Note that the manufacturing method of the present disclosure is not limited to the conditions used in the following examples.

<Sample preparation>
Samples of Examples 1 to 8 and Comparative Examples 1 to 7 were prepared as follows.

Examples 1 to 8 and Comparative Examples 1 to 5
5.0 kg of FeSO ₄ · 7H ₂ O was mixed and dissolved in 2.0 kg of pure water. 0.49 kg of Sm ₂ O ₃ , 0.74 kg of 70% sulfuric acid, and 0.035 kg of La ₂ O ₃ were further added and thoroughly stirred to completely dissolve. Next, pure water was added to the obtained solution, and the final Fe concentration was adjusted to 0.726 mol / L and the Sm concentration to 0.112 mol / L to obtain a SmFeLa sulfuric acid solution.

[Precipitation step]
The entire amount of the prepared SmFeLa sulfuric acid solution was dropped into 20 kg of pure water kept at a temperature of 40°C over a period of 70 minutes from the start of the reaction while stirring, and at the same time, 15% ammonia solution was dropped to adjust the pH to 7 to 8. This resulted in a slurry containing SmFeLa hydroxide. The obtained slurry was washed with pure water by decantation, and the hydroxide was subjected to solid-liquid separation. The separated hydroxide was dried in an oven at 100°C for 10 hours.

[Oxidation process]
The hydroxide obtained in the precipitation step was calcined in air at 1000° C. for 1 hour. After cooling, red SmFeLa oxide was obtained as a raw material powder.

[Pretreatment process]
100 g of SmFeLa oxide was placed in a steel container so that the bulk thickness was 10 mm. The container was placed in a furnace, and after reducing the pressure to 100 Pa, the temperature was raised to the pretreatment temperature of 850° C. while introducing hydrogen gas, and the temperature was maintained for 15 hours. The oxygen concentration was measured by non-dispersive infrared absorption (ND-IR) (EMGA-820 manufactured by Horiba, Ltd.) and found to be 5 mass %. This revealed that the oxygen bonded to Sm was not reduced, and 95% of the oxygen bonded to Fe was reduced to obtain a black partial oxide.

[Reduction process]
60 g of the partial oxide obtained in the pretreatment process and 19.2 g of metallic calcium having an average particle size of about 6 mm were mixed and placed in a furnace. After evacuating the furnace, argon gas (Ar gas) was introduced. The temperature was raised to 1090° C. and held for 45 minutes, and then cooled to obtain SmFe powder particles.

[Nitriding process]
Subsequently, the temperature inside the furnace was cooled to 100° C., and then the furnace was evacuated to a vacuum, and while introducing nitrogen gas, the temperature was increased to a first temperature of 430° C. and held for 3 hours. Subsequently, the temperature was increased to a second temperature of 500° C. and held for 1 hour, and then cooled to obtain a mass product containing magnetic particles.

[Post-processing process]
The aggregated product obtained in the nitriding step was poured into 3 kg of pure water and stirred for 30 minutes. After standing, the supernatant was drained by decantation. The process of pouring into pure water, stirring and decantation was repeated 10 times. Then, 2.5 g of 99.9% acetic acid was poured into the product and stirred for 15 minutes. After standing, the supernatant was drained by decantation. The process of pouring into pure water, stirring and decantation was repeated twice.

[Acid treatment step]
A 6% aqueous hydrochloric acid solution was added to 100 parts by mass of the powder obtained in the post-treatment process so that the hydrogen chloride content was 4.3 parts by mass, and the mixture was stirred for 1 minute. After standing, the supernatant was drained by decantation. The mixture was poured into pure water, stirred, and decanted twice. After solid-liquid separation, the mixture was vacuum dried at 80°C for 3 hours to obtain SmFeN powder having a composition of Sm _9.2 Fe _77.1 N _13.59 La _0.11 .

The SmFeN powder was packed into a sample container together with paraffin wax, and the paraffin wax was melted in a dryer, after which the easy axis of magnetization was aligned in an orientation magnetic field of 16 kA/m. This magnetically oriented sample was pulse magnetized in a magnetizing magnetic field of 32 kA/m, and the magnetic properties were measured at room temperature using a VSM (vibrating sample magnetometer) with a maximum magnetic field of 16 kA/m, revealing a residual magnetization of 1.44 T and a coercive force of 750 kA/m.

The SmFeN powder obtained as described above was classified to obtain a powder of the first particle group and a powder of the second particle group. The powder of the first particle group and the powder of the second particle group were then mixed using an N-type mixer to obtain a magnetic powder. The particle size distribution of each of the first particle group and the second particle group was as shown in Table 1. The ratio of the total volume of the first particle group to the total volume of the second particle group (total volume of the first particle group:total volume of the second particle group) was as shown in Table 1-1. Table 1-1 also lists the particle remanence magnetization σr of each of the first particle group and the second particle group.

A metallic zinc powder was prepared as the modifier powder. The metallic zinc powder had a _D50 of 0.5 μm and a purity of 99.5% by mass.

The magnetic powder (powder of the first particle group and powder of the second particle group) and the modifier powder were mixed to obtain a mixed powder. The amount of metallic zinc mixed in the total mixed powder was 5 mass%.

The mixed powder was compression molded in a magnetic field to obtain a magnetically molded body. The pressure for compression molding was 50 MPa. This pressure was applied for 1 minute. The magnetic field applied was 1600 kA/m. The compression molding was performed in a nitrogen atmosphere.

The magnetic field compacts were pressure sintered. For the samples of Examples 1 to 6 and Comparative Examples 1 to 5, pressure sintering was performed in an argon gas atmosphere (97,000 Pa) using a high-frequency induction coil. For the samples of Examples 7 to 8, pressure sintering was performed in a nitrogen gas atmosphere (10,000 Pa) using discharge plasma heating (SPS method). For all samples, the sintering temperature was 380°C, the sintering pressure was 1,000 MPa, and the sintering pressure was applied for 5 minutes.

Comparative Examples 6 to 7
A sample of Comparative Example 6 was prepared in the same manner as in Example 1, and a sample of Comparative Example 7 was prepared in the same manner as in Example 3, except that only powder of the first particle group was used as the magnetic powder and no powder of the second particle group was used.

Example 9
A sample of Example 9 was prepared in the same manner as in Example 4, except that a coating containing phosphoric acid was formed on the particle surface of the second particle group, and the sintered body after pressure sintering was heat-treated. The coating containing phosphoric acid was formed before mixing the SmFeN powder (powder of the first particle group and powder of the second particle group) with the modifier powder. That is, the SmFeN powder was classified into the first particle group and the second particle group, a coating containing phosphoric acid was formed on the particle surface of the second particle group, and the powder of the first particle group as classified, the powder of the second particle group on which the coating containing phosphoric acid was formed, and the modifier powder were mixed.

To form the coating containing phosphoric acid, a dispersion process and a surface treatment process were carried out as preparatory processes prior to the phosphoric acid treatment process. Details of the dispersion process, surface treatment process, and phosphoric acid treatment process are as follows.

[Dispersion process]
The powder of the second particle group and the media were placed in a container so that the powder of the second particle group was 5 volume % and the media (iron-core nylon media, diameter 10 mm, Vickers constant of nylon coating 7, specific gravity 7.48 g/cm3) was 60 volume % relative to the volume of the container used for the vibration mill. The mixture was dispersed in a nitrogen atmosphere for 60 minutes using the vibration mill to obtain an intermediate powder.

[Surface treatment process]
The obtained intermediate powder was put into pure water and stirred for 1 minute. An acid solution was added to this slurry to perform etching. The acid solution used was a hydrochloric acid solution. 50 g or more of 5% hydrochloric acid was added to 100 g of the intermediate powder while stirring. Then, after confirming that the pH was 3 or more, decantation was performed until the electrical conductivity of the slurry was 100 μS/cm or less.

[Phosphating process]
A phosphoric acid solution was added to the obtained slurry. The phosphoric acid solution was added in an amount of 1 mass% as _PO4 relative to the solid content of the particles of the second particle group. After stirring for 5 minutes and performing solid-liquid separation, vacuum drying was performed at 120°C for 3 hours to obtain a powder of the second particle group having a coating film containing phosphoric acid.

The sintered body after pressure sintering was heat treated under the conditions shown in Tables 2-1 and 2-2. Table 2-1 also lists the particle remanence _σr and particle coercivity Hc of the first particle group and the second particle group. In Table 2-1, the "coating containing phosphoric acid" is expressed as the "phosphate coating".

Example 10
A sample of Example 10 was prepared in the same manner as in Example 9, except that a coating containing phosphoric acid was not formed on the particle surfaces of the second particle group.

"evaluation"
The density and magnetic properties of each sample were measured. The density was measured by the Archimedes method. The magnetic properties were measured using a vibrating sample magnetometer (VSM). For the samples of Example 1, Comparative Example 3, and Comparative Example 6, the cross section of the sample was polished, and the polished surface was subjected to structural observation using a scanning electron microscope (SEM).

The evaluation results are shown in Tables 1-1 to 1-2, Tables 2-1 to 2-2, and Figures 5 to 9. Figure 5 is a graph showing the relationship between _d2 / _d1 and density. Figure 6 is a graph showing the relationship between _d2 / _d1 and remanent magnetization Br. Figure 7 shows an SEM image of the sample of Example 1. Figure 8 shows an SEM image of the sample of Comparative Example 3. Figure 9 shows an SEM image of the sample of Comparative Example 6.

As shown in Tables 1-1 and 1-2 and FIGS. 5 and 6, it can be seen that the samples of Examples 1 to 8 in which _d1 and _d2 satisfy a predetermined relationship and the total volume of the first particle group: the total volume of the second particle group is within a predetermined range have high density and, as a result, excellent residual magnetization.

On the other hand, in the samples of Comparative Example 1 and Comparative Example 2, even if the total volume of the first particle group: the total volume of the second particle group is within a predetermined range, _d2 / _d1 does not satisfy the predetermined relationship, so the density is low and, as a result, the residual magnetization is low. In the samples of Comparative Examples 3 to 5, although _d2 / _d1 satisfies the predetermined relationship, the total volume of the first particle group: the total volume of the second particle group is not within a predetermined range, so the density is low and, as a result, the residual magnetization is low. In addition, in the samples of Comparative Example 6 and Comparative Example 7, only the powder of the first particle group was used, and no powder of the second particle group was used, so the density is low and, as a result, the residual magnetization is low.

For example, the density of the sample in Example 1 is higher than the densities of the samples in Comparative Example 3 and Comparative Example 6, which can be seen from the fact that the dark areas (voids) in the SEM image of the sample in Example 1 (Figure 7) are smaller than the dark areas in the SEM images of the samples in Comparative Example 3 and Comparative Example 6 (Figures 8 and 9).

For Examples 9 and 10, _d1 and _d2 satisfy a predetermined relationship, and the total volume of the first particle group: the total volume of the second particle group is within a predetermined range, so that the density is high and, as a result, the residual magnetization is excellent. In addition, in Example 9, a coating containing phosphoric acid is formed on the particle surface of the second particle group, whereas in Example 10, a coating containing phosphoric acid is not formed on the particle surface of the second particle group. Therefore, it can be understood that the sample of Example 9 has a large Hk at 120 ° C. and is excellent in squareness at high temperatures compared to the sample of Example 10.

These results confirm the effectiveness of the disclosed method for producing rare earth magnets and the rare earth magnets obtained using the method.

10 SmFeN powder particles (magnetic particles)
11 First particle group 12 Second particle group 20 Modifier 30 Modifier phase 100 Rare earth magnet obtained by the manufacturing method of the present disclosure 200 Rare earth magnet obtained by a conventional manufacturing method

Claims (8)

A magnetic powder is prepared, which contains Sm, Fe, and N and has a magnetic phase at least partly having a crystal structure of either Th ₂ Zn ₁₇ type or Th ₂ Ni ₁₇ type;
Preparing a modifier powder containing at least one of metallic zinc and a zinc alloy;
mixing the magnetic powder and the modifier powder to obtain a mixed powder;
Compressing and molding the mixed powder in a magnetic field to obtain a magnetically molded body; and pressure-sintering the magnetically molded body to obtain a sintered body.
Including,
the magnetic powder includes a first particle group and a second particle group,
The particle size distribution _D50 of the first particle group is expressed in terms of _d1 μm, and the particle size distribution _D50 of the second particle group is expressed in terms of _d2 μm;
The _d1 and _d2 satisfy the relationship of 0.350≦ _d2 / _d1 ≦0.500 ,
a ratio of a total volume of the first particle group to a total volume of the second particle group (total volume of the first particle group:total volume of the second particle group) is in the range of 9:1 to 4:1 ,
_The d1 is 3.0 to 3.7 μm, and
_The d2 is 1.4 to 1.8 μm;
A method for manufacturing rare earth magnets. 2. The method for producing a rare earth magnet according to claim 1 , wherein the modifier powder has a _D50 of 0.1 to 12.0 μm, and the content of zinc in the modifier powder is 1 to 30 mass % relative to the mixed powder. 3. The method for producing a rare earth magnet according to claim 1 , wherein the mixed powder is compression molded at a pressure of 10 to 1500 MPa. The method for producing a rare earth magnet according to any one of claims 1 to 3 , wherein the magnetic compact is pressure sintered at a pressure of 100 to 2000 MPa and a temperature of 300 to 430°C for 1 to 30 minutes. A magnetic powder is prepared, which contains Sm, Fe, and N and has a magnetic phase at least partly having a crystal structure of either Th ₂ Zn ₁₇ type or Th ₂ Ni ₁₇ type;
Preparing a modifier powder containing at least one of metallic zinc and a zinc alloy;
mixing the magnetic powder and the modifier powder to obtain a mixed powder;
The mixed powder is compressed in a magnetic field to obtain a magnetically compacted body; and
sintering the magnetic field compact under pressure to obtain a sintered body;
Including,
the magnetic powder includes a first particle group and a second particle group,
The particle size distribution _D50 of the first particle group is expressed in terms of d1 μm _, and the particle size distribution _D50 of the second particle group is expressed in terms of d2 _μm ;
_The d1 and d2 _satisfy the relationship of 0.350≦d2 _/ d1 _≦ 0.500, and
A method for producing a rare earth magnet, wherein a ratio of a total volume of the first particle group to a total volume of the second particle group (total volume of the first particle group:total volume of the second particle group) is in the range of 9:1 to 4:1,
forming a modification-suppressing coating on the particle surfaces of the second particle group before the pressure sintering; and
heat-treating the sintered body to promote modification of the particle surfaces of the first particle group;
The method for producing a rare earth magnet further comprises: The above d ₁ is 3.0 to 3.7 μm, and ₂ The method for producing a rare earth magnet according to claim 5, wherein the average particle size is 1.4 to 1.8 μm. The method for producing a rare earth magnet according to claim 5 or 6 , wherein the modification-suppressing coating contains phosphoric acid. The method for producing a rare earth magnet according to any one of claims 5 to 7 , wherein the sintered body is heat treated at a temperature of 350 to 410°C.

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